PLEASE NOTE: You may already have a version of Windows Live Movie Maker on your computer. However, the older versions are difficult to work with and do not make it easy to add both narration and music. Please download the newest version of Windows Live Movie Maker here even if you already have the old version.
iMovie is basically the Apple counterpart to Windows Live Movie Maker. If you have a Mac, iMovie should come pre-installed on it. It never hurts to have the latest version, though. Please note that the download price is $14.99.